Phosphorescent devices

ABSTRACT

Incoherent light sources depending on phosphors which may simultaneously emit at more than one wavelength are provided with multiple dielectric coatings to suppress a portion of the emission and thereby enhance the remainder. The use of such coatings with frequency up-converting phosphors as well as downconverting phosphors is described.

United States Patent Geusic et al.

PHOSPHORESCENT DEVICES lnventors: Joseph Edward Geuslc, BerkeleyHeights; Frederick William Ostermayer, Jr., New Providence; Le GrandGerard Van Ultert, Morris Township, Morris County, all of Assignee: BellTelephone Laboratories, Incorporated,

Murray Hill, NJ.

Filed: Jan. 19, 1970 Appl. No.: 4,006

u.s. Cl. ..250/71 R, 250/77 Int. Cl. ..F2lk 2/00 Field oi Search..250/71 R, 77, 86; 350/311 [451 Apr. 4, 1972 [56] References CitedUNITED STATES PATENTS 3,484,606 12/1969 Masi ..250/71 R 2,904,689 9/1969Masi et al ..250/77 X Primary Examiner-Archie R. Borchelt AssistantExaminer-Davis L. Willis Attorney-R. J. Guenther and Edwin B. Cave [57]ABSTRACT Incoherent light sources depending on phosphors which maysimultaneously emit at more than one wavelength are provided withmultiple dielectric coatings to suppress a portion of the emission andthereby enhance the remainder. The use of such coatings with frequencyup-converting phosphors as well as down-converting phosphors isdescribed.

3 Claims, 6 Drawing Figures EXClTATlON (2) RADIATION -LAAJVVVUPHOSPHORESCENT DEVICES BACKGROUND OF THE INVENTION 1. Field of theInvention The invention is concerned with incoherent light sourcesutilizing phosphor emission.

2. Description of the Prior Art Incoherent light sources based onphosphor emission are already in prevalent use and many new uses arecontemplated. Such sources depend upon a variety of pump means as, forexample, electron bombardment in cathode ray tubes; d.c. electricbiasing in junction devices, such as those using gallium arsenide; andlight pumping as in a variety of display devices. The latter categoryincludes higher frequency pumping in most common devices and lowerfrequency pumping as in second photon devices. See Bulletin of theAmerican Physical Society, Series ll, Vol. 13, No.4, p. 687, PaperI-IK7.

Phosphor materials are of many types, some inorganic, some organic; someemit over rather narrow bandwidths, some over broad bandwidths.

In any of the foregoing categories, a situation may arise in which partof the pump energy is converted to undesired emission. This undesiredemission may be within or without the visible spectrum. A specificexample of recent concern has to do with second photon sources utilizinglong wavelength pumps. In one such example, a forward biased GaAs diodeis used to pump a rare earth-containing, second photon phosphor toproduce visible emission. Whereas such devices operate efficiently atgreen and red wavelengths, difficulty has been encountered infabricating an efficient blue source. In this particular example, a bluesource is desired for the construction of a three-color display system.While thulium-containing materials (the initial absorption functionbeing performed by ytterbium) emit blue light when pumped by theinfrared emission from the diode, a significant part of the pump energyis converted to a different wavelength of near infrared emission. As aresult, the efficiency of conversion to blue is diminished. Many othersimilar examples exist.

A further complication resulting in inefficiency in phosphorescentdevices is concerned with inefficient utilization of pump energy. Inlight-pumped devices, absorption coefficients for different involvedwavelengths may dictate different optimum thicknesses for emission andfor pump energy. Under some circumstances, for example, dimensionoptimization for emission may result in inefficient absorption of pumpenergy.

Problems similar to many of the foregoing were a deterrent to thedevelopment of the laser. The problem there was largely one of absorbingsufficient pump energy to create the required population inversion.Resort was had to layered structures of various dielectric films allindividually transparent to wavelengths of concern. Choice of thicknessof two or more materials of appropriate refractive indices results inconstructive and destructive interference at selected wavelengths.

This approach has permitted the design of a cavity which is essentiallytotally resonant for the pump frequency. Energy of the wavelength ofconcern may also be resonatedso, as to give the required statisticalnumber of passes for desired operation. See Applied Optics and OpticalEngineering, ed. R. Kingslake, Academic Press, New York, 1965, Ch. 8.

SUMMARY OF THE INVENTION In accordance with the invention, multilayeredcoatings of transparent materials of critical thickness and refractiveindices partially or totally encompassing phosphor materials result insuppression of energy of one or more wavelengths while permittingtransmission of energy of one or more other wavelengths. This is ageneral solution which results in improvement of efficiency ofincoherent phosphorescent devices in any of the classes set forth above.In certain embodiments, pumping efficiency is improved by preventingescape of part of the pump energy or even by creating resonantconditions for such pump energy. In the preferred embodiment,significant improvement in emission is brought about by suppression ofone or more emission wavelengths to enhance at least one otherwavelength in phosphors having relevant emission spectra.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is an energy level diagram inordinate units of wavenumbers for an appropriate second-photon phosphorsystem illustrative of systems suitable for improvement in accordancewith the inventive principles;

FIG. 2 is a sectional view of a structure showing improved emissionefficiency in accordance with the invention;

FIG. 3 on coordinates of transmittance in percent, and wavelength inmicrons illustrates the relationship of these coordinates for aparticular layered structure;

FIG. 4, in ordinate units of wavenumbers is an energy diagramillustrating a down-converting phosphor system with multiple emissionlines, the efficiency of which may be improved in accordance with theinvention;

FIG. 5 is a sectional view of a phosphor layer dielectrically coated inaccordance with the invention; and

FIG. 6 is a sectional view of a portion of a structure alternative tothat of FIG. 5.

DETAILED DESCRIPTION The invention has been generally described. Thestate of the concerned arts is such that further description isunnecessary to enable the person skilled in the art to practice theinvention. Suitable dielectric materials, relevant dielectric layerparameters including refractive indices and thicknesses foraccomplishment of suppression and transmission as desired are availablein the literature. See'for example Applied Optics and OpticalEngineering, ed. by R. Kingslake, Academic Press (1965 Vol.11, Ch. 8.

For illustrative purposes, a detailed description is set forth in termsof the ytterbium-thulium, second-photon phosphor. This particular systemis of interest as a blue light source, for example, as an indicatorlight or a portion of a display screen with light pumping at a suitableinfrared wavelength. Since absorption is relatively narrow, thismaterial is particularly suitable for use with a narrow band emittingpump such as a laser or a forward biased incoherent diode. The primeexample of the latter at this writing is the gallium arsenide diode.

1. Drawing FIG. 1. In the ytterbium-thulium system (suitable hostsinclude yttrium fluoride), infrared excited blue emission is produced bya three-step sequential excitation. The efficiency of the infraredexcited blue emission from level 3 (all levels encircled on the figure)is approximately 0.1 percent blue power out intrared power in Atpresent, the blue emission is limited to this low value becausesignificant emission at 8,000 A. from level 2 occurs. In fact, theemission from 8,000 A. is from 4 to 10 percent efficient. A technique toimprove the blue emission at the expense ofthe 8,000 A. emission is toprovide a reflective coating on the phosphor so as to effectivelyincrease the radiative lifetime of level 2, thus increasing theprobability of excitation of atoms to level 3 as compared to theprobability of the 8,000 A. radiative transition. In Tm, the 8,000 A.transition occurs to the ground state; and in this case, if a coating ofreflectivity R is used, the effective radiative lifetime can beincreased to where -r is the normal radiative lifetime of the Tm "2level. Since with multilayer coatings a reflectivity of greater thanpercent is easily achievable, emission at 4,800 A. (blue) is increasedby at least a factor of 10.

Several methods of entrapping the 8,000 A. radiation to improve the blueemission are discussed in FIGS. 2 and 3. In FIG.

2 phosphor 1 such as YF zYbjlm is in the form of a thin transparentcoating on the diode 2 which may be Si-GaAs. The dome surface 3 of thediode and the outer surface 4 of the phosphor have been coated with amultilayer coating which is reflective at 8,000 A. and transparent at4,800 A.

A fifteen-layer coating which can be-used is represented in FIG. 3. Thecoating consists of a thirteen-layer, l/4 0.57 1,), high and low indexstack in which the high index layer H ZnS and the low index layer L MgFOn either end, a 56). layer (H/2) of the high index material is used.The general characteristic of such a coating is also shown. If also thecoatings are partially reflective at the pump frequency (093p for GaAsdiodes) one can get an even further enhancement because the intensity ofthe 0.93 p. radiation in the phosphor is effectively increased by afactor proportional to the number of internal reflections. Theenhancement of the efficiency of conversion to blue light (4,880 A.) isproportional to the N-l power of the 0.93;. intensity where N is thenumber of sequential photons involved providing saturation effects havenot been reached. N=3 for 0.48011. emission.

Enhancement at 8,000 A is discussed. The coating is highly reflecting at4,880 A if the layers of the same dielectric structure described aboveare VAX at 2,800 A. or 700 A. thick. Such a coating reflects 4,800 A.and transmits 8,000 A. and 0.93u. Thus the Tm phosphor can be used topump YAG:Nd which absorbs at 8,000 A. without undue loss as blueemission (4,880 A.). Normal operation is 300 Amperes/cm in a GaAs diode.

Alternative ions emitting in the visible are Er, Ho. Devices are againprovided with coatings that reflect at all emission energies save theone desired. It is important to provide suitable reflection particularlyfor undesirable emissions having short intrinsic radiative lifetimes.

Suitable host materials and other considerations germane to the designof efiicient light sources of the type described in conjunction withFIG. 2 have been set forth elsewhere, see Applied Physics Letters,Volume 15, No. 2, pages 48 to 54. Host materials may be simple fluoridesor more complex media shown to enhance operation inaccordance with avariety of mechanisms.

The energy diagram of FIG. 4 is illustrative of a more conventionalphosphor which emits at several wavelengths A A and A all longer thanthe pumping wavelength. If any one of these fluorescences, say A ispreferred, emission at that wavelength is improved by the suppression ofemission from the phosphor at the undesired wavelengths A, and )0, usingmultilayer coatings on the phosphor which are highly reflective at theundesired wavelengths and transmitting at the desired wavelength. Whilethe concept and the diagramsare general and apply to a large number ofconventional phosphors, a specific example is a phosphor containing theactive ion Er in which case k, is a band of wavelengths from 0.5 0.4g.and A,=0.55u, ).,=0.65u and A =0.82p.. For this case, the pump may be aconventional Hg-Arc source.

FIG. 5 depicts a phosphor layer 10 covered by coatings 11 and 12.Coating materials are selected in accordance with the considerations setforth above.

In FIG. 6, the phosphor material 15 is particulate and each particle iscoated with multiple layers 16 to accomplish the end described. Whilepresent techniques do not produce coatings of the thickness uniformitywhich may be accomplished on massive smooth surfaces, procedures areavailable for producing coatings which, while they may not optimize,nevertheless improve emission efficiency. Such techniques includeevaporation, sputtering and various other deposition techniques.

2. Design Requirements The general requirement of the invention is thatat least one emitting surface of a phosphor be contacted by at least twolayers of materials of differing refractive indices so chosen as tounequally suppress a portion of the spectrum relative to another suchportion. Suitable materials are necessarily transparent to all concernedwavelengths, it being considered that an absorption of 5 percent at anyconcerned wavelength is the maximum permitted. The number of layers,their indices and thickness, all depend on the particular circumstancesinvolved.

It is known that the applicable principles are those of conventionalfilter design. Where it is desired to suppress or transmit a relativelybroad bandwidth to a relatively uniform degree, a large number, forexample fifteen or more layers may be required. In less sophisticateddevices where it may suffice merely to suppress one or more relativelynarrow bands and/or where flat response is of little consequence, asmaller number of layers, as few as two, may suffice.

We claim:

1. Incoherent phosphorescent emission source comprising a phosphoradapted to at least partially transmit electromagnetic radiation ofdifferent wavelengths, characterized in that said phosphor is providedwith a medium at least partially encompassing said phosphor, said mediumconsisting essentially of at least two successive layers, said layersbeing of such thicknesses and having such refractive indices as tosuppress one of the said wavelengths relative to the other in which saidphosphor is of such nature as to produce at least one wavelength whichis shorter that that of a pump.

2. Source of claim 1 in which the wavelength of the said pump is in theinfrared spectrum.

3. Source of claim 2 in which the said pump is a forward biased, galliumarsenide diode.

1. Incoherent phosphorescent emission source comprising a phosphoradapted to at least partially transmit electromagnetic radiation ofdifferent wavelengths, characterized in that said phosphor is providedwith a medium at least partially encompassing said phosphor, said mediumconsisting essentially of at least two successive layers, said layersbeing of such thicknesses and having such refractive indices as tosuppress one of the said wavelengths relative to the other in which saidphosphor is of such nature as to produce at least one wavelength whichis shorter that that of a pump.
 2. Source of claim 1 in which thewavelength of the said pump is in the infrared spectrum.
 3. Source ofclaim 2 in which the said pump is a forward biased, gallium arsenidediode.